Central lighting systems wherein a single light source is used to supply light output to a number of spaced apart locations using a plurality of optical fibers have been proposed for a number of various applications including automotive, display and home lighting. For instance, U.S. Pat. No. 4,958,263 issued to Davenport et al on Sep. 18, 1990 and assigned to the same assignee as the present invention, discloses the use of a central lighting system for an automotive application. The light source for such a central lighting system must be capable of providing a light output having high brightness characteristics so as to insure sufficient light output at the spaced apart locations. U.S. patent application Ser. No. 07/858,906 entitled "High Brightness Discharge Light Source" filed contemporaneously herewith in the name of Allen et al and assigned to the same assignee as the present invention, discloses a light source having a brightness characteristic measured in excess of 50,000 lumens per centimeter squared. A light source having this level brightness output and which would be used in conjunction with a reflector arrangement for focussing the light output to an optical focal point at which the optical fibers are disposed, can cause physical damage to the ends of the optical fibers placed at such optical focal point. One theory for the cause of such damage is that the image of the anode of the discharge light source contains more of an infrared radiation component than does the actual arc discharge and that such IR radiation is the cause of the damage to the optical fiber ends. Accordingly, it would be advantageous that in a central lighting system having a high brightness light source, means could be provided that would prevent damage to the ends of the optical fibers that may be caused by IR radiation or some other source of heat or radiation.
Central lighting systems are utilized in applications where it is necessary to distribute the light output to a relatively large number of different locations thereby necessitating the use of a large number of individual optical fibers. Given that there is only a limited space in which to dispose the ends of the optical fibers for receiving the light output, it would be advantageous if as large a number of small diameter fibers as possible could be accommodated or, conversely that if larger diameter optical fibers were used, that as many as possible could be packed within the space provided for receiving the light output of the light source. This concept is typically referred to as the packing fraction of the optical coupling arrangement. One arrangement for achieving a high packing fraction is disclosed in U.S. Pat. No. 5,058,985 issued to Davenport et al on Oct. 22, 1991 and assigned to the same assignee as the present invention. In this patent, it is disclosed that a combination of compressible and non-compressible fibers are disposed within a sleeve member sized so that the compressible fibers are pressed in a conforming shape around the non-compressible fibers thereby achieving a relatively uniform cross-sectional surface area at the optical coupler input. Although such an arrangement has proven effective in increasing the packing fraction for a grouping of optical fibers, such an arrangement could be susceptible to damage caused by heat or IR radiation.
It has also been found that the spacing that exists between the fibers as caused by the cladding disposed around each individual optical fiber, detracts from the useful space for coupling and moreover, establishes a separate position for each of the fibers at the face of the optical coupling device. The separate positioning characteristic has the inherent disadvantage that there is a distinct index of refraction for each fiber that results in the reflection of light at the walls of each fiber. When one considers that the light image received by each of the individual fibers will only see the color and intensity of light output from the light source at its specific position at the face of the optical coupler, the result is a variance in color and intensity properties at the optical fiber outputs. For optical fibers in the middle of the optical coupler, the color characteristics and the intensity of the light received will differ from that of the optical fiber located at the outer perimeter of the optical coupler. U.S. Pat. No. 4,662,714 issued to Mori on May 5, 1987 discusses stripping the cladding from a portion of the optical fibers; such disclosure however makes no reference to the desirability of providing for a mixture of the light so as to achieve uniform color and intensity characteristics over each of the individual optical fibers. Accordingly, it would be advantageous for an optical coupling device which accommodates a plurality of individual optical fibers to provide a means for mixing the color and intensity properties so that each of the fibers transmits to its end destination, light output having uniform characteristics regarding color and intensity.